Stability of KcsA tetramer depends on membrane lateral pressure.

The potassium channel KcsA forms an extremely stable tetramer. Despite this high stability, it has been shown that the membrane-mimicking solvent 2,2,2-trifluoroethanol (TFE) can induce tetramer dissociation [Valiyaveetil, F. I., et al. (2002) Biochemistry 41, 10771-7, and Demmers, J. A. A., et al. (2003) FEBS Lett. 541, 69-77]. Here we have studied the effect of TFE on the structure and oligomeric state of the KcsA tetramer, reconstituted in different lipid systems. It was found that TFE changes the secondary and tertiary structure of KcsA and that it can dissociate the KcsA tetramer in all systems used. The tetramer is stabilized by a lipid bilayer as compared to detergent micelles. The extent of stabilization was found to depend on the nature of the lipids: a strong stabilizing effect of the nonbilayer lipid phosphatidylethanolamine (PE) was observed, but no effect of the charged phoshosphatidylglycerol (PG) as compared to phosphatidylcholine (PC) was found. To understand how lipids stabilize KcsA against TFE-induced tetramer dissociation, we also studied the effect of TFE on the bilayer organization in the various lipid systems, using (31)P and (2)H NMR. The observed lipid dependency was similar as was found for tetramer stabilization: PE increased the bilayer stability as compared to PC, while PG behaved similar to PC. Furthermore, it was found that TFE has a large effect on the acyl chain ordering. The results indicate that TFE inserts primarily in the membrane interface. We suggest that the lipid bilayer stabilizes the KcsA tetramer by the lateral pressure in the acyl chain region and that this stabilizing effect increases when a nonbilayer lipid like PE is present.